1. Field of the Invention
This invention relates to a novel ion charging apparatus wherein ions are generated in a housing and passed through a narrow conducting exit slit in order to uniformly charge a charge receptor.
Corona charging of xerographic photoreceptors has been disclosed as early as U.S. Pat. No. 2,588,699. It has always been a problem that current levels for practical charging require coronode potentials of many thousands of volts, while photoreceptors typically cannot support more than 1000 volts surface potential without dielectric breakdown.
One attempt at controlling the uniformity and magnitude of corona charging is U.S. Pat. No. 2,777,957 which makes use of an open screen as a control electrode, to establish a reference potential, so that when the receiver surface reaches the screen voltage, the fields no longer drive ions to the receiver, but rather to the screen. Unfortunately, a low porosity screen intercepts most of the ions, allowing a very small percentage to reach the intended receiver. A more open screen, on the other hand, delivers charge to the receiver more efficiently, but compromises the control function of the device.
Other methods exist for trying to obtain uniform charging from negative charging systems such as dicorotron charging devices as shown in U.S. Pat. No. 4,086,650 that includes glass coated wires and large specialized AC power supplies. A self cleaning charging unit is disclosed in U.S. Pat. No. 4,841,146 that includes an insulating housing with conductive plates attached to a bottom surface thereby forming a slit for the passage of ions from a corona source positioned within the housing toward a receptor surface. The housing includes insulating wedges positioned above and in contacting relation with each conductive plate in order to focus additional ions toward the center of the slit.
Various ion generating devices are available for printing or charging purposes. For example, in U.S. Pat. No. 4,463,363 there is taught a D.C. air breakdown form of ion generator. In U.S. Pat No. 4,524,371 a fluid jet assisted ion projection printing apparatus is disclosed that includes a housing having ion generation and ion modulation regions. A bent path channel, disposed through the housing, directs transport fluids with ions entrained therein adjacent an array of modulation electrodes which control the passage of ion beams from the device. Emission of charged particles in U.S. Pat. No. 4,155,093 is accomplished by extracting them from a high density source provided by an electrical gas breakdown in an alternating electrical field between two conducting electrodes separated by an insulator. A corona discharge unit is used in conductive toner transfer in a copier in U.S. Pat. No. 4,174,170. The corona discharge unit includes a slit to permit transfer of conductive toner particles onto a copy paper charged by the corona unit. A corona wire in the unit is surrounded by a shield. U.S. Pat. No. 3,396,308 discloses a web treating device for generating a flow of ionized gas. This device includes an opening through which the gas is directed towards a receptor surface. An elongated hollow hosing 11 has tapered sides 14 terminating in a pair of lips 15 which form a narrow and elongated slot 16. U.S. Pat. Nos. 3,598,991 and 4,100,411 show electrostatic charging devices including a corona wire surrounded by a conductive shield. In U.S. Pat. No. 3,598,991, a slit 13 is formed in the shield to allow ions to flow from wire 12 to a photoconductive surface 2 to deposit an electric charge thereon. In U.S. Pat. No. 4,100,411, a pair of lips 16 and 17 define a corona ion slit 18. Japanese Patent Document No. 55-73070 discloses a powder image transfer type electrostatic copier that includes a corona discharge device having a slit in a shield plate. In Japanese Patent Document No. 54-156546 a corona charge is shown having a plurality of grating electrodes in the opening part of a corona shield electrode. These devices have not been entirely satisfactory in that they are costly, some of them are hard to fabricate and most are inefficient.
In addition, the problem of uniformity of charge is more pronounced when migration imaging is attempted using Verde film that is disclosed in U.S. Pat. No. 5,411,825. In contrast to typical copier/printer speeds which are nearer to 4 inches per second, the process speed for Verde film is very slow and at times less than 4 inches per minute. Consequently, Verde film light sensitivity requires approximately 3.5 times less light for exposure than some photoreceptors. With Verde film traveling at 4 inches per minute beneath the corona, fogging as a result of exposure from corona generated light is a major problem.
Accordingly, a charging apparatus is provided for use in any of the various printing and imaging processes. The light blocking ion charging apparatus of the present invention overcomes the above-described problems and disadvantages of conventional charging devices.
Specifically, this invention in one embodiment provides a charging device with a light eclipsing element juxtaposed between a corona source and a charge receptor. The light eclipsing element blocks the corona generated light without unduly affecting the performance of the charging device.
In another embodiment, the charging apparatus of the present invention includes a corona source and a conducting grid of parallel screen wires on the like spaced between the conducting screen and a charge receptor. Eclipsing rods or wires are positioned between the corona source and conducting screen and spaced so that their shadows fall in the spaces between the grid of parallel screen wires to thereby intercept corona light while allowing the flow of ions from the corona source to the charge receptor.
In yet another embodiment, the charging apparatus of the present invention includes a corona generating source, an eclipse element along with two conductive, semi-conductive, or insulating wedges that focus ion emanating from the corona generating source toward a pair of conductive slit forming electrodes. The eclipse element prevents light emitted from the corona generating source from exiting the slit.